I am relative new to photography so I am starting to buy camera gear. I already have a Nikon D3300 with stock lens. I've been told that if I am not using a speedlight flash I am just using a 60% of the camera capability. True or not , I still think it is a good idea to get one as I start looking at some Internet videos about filling shadows and balancing potential two exposures.

Anyway, I was looking at Nikon SB-700 and SB-910, so have these questions:
- Are both of them compatible with the D3300 model?
- Should I start with the SB-700 and move on when I eventually get a better camera body model?
- Is it a real difference between the two models that justify paying almost double the price?

I am at a begginer level, so not sure I should start with the basic or think about this on the long term. Any help is appreciated !!!

I don't know about "only 60%", but a proper speedlight does increase your capability significantly. There is definitely enough difference between the '700 and '910 to justify the price difference, but the question is really, do you need that increased capability? You probably don't now; you may in the future, but probably not now, so if you have the extra cash, sure stump up for the '910, but if not, go for the '700, and get the '910 later (Speedlights are like salted peanuts; you're never really satisfied with just one).

I own 4 SB800 and 4 AB800 and a ton of light modifiers. I have a room just for lighting equipment.

I say to you. Learn your camera and how to shoot AND edit to a point before you add another complicated part of the equation. Lights have a place in the art but until you have mastered a single exposure, adding another in the mix is going to be frustrating as hell. If you master single exposure technique first, adding a light will be a natural progression.